The role of biological processes in reducing both odor impact and pathogen content during mesophilic anaerobic digestion

Mesophilic anaerobic digestion (MAD) produces renewable energy, but it also plays a role in reducing the impact of digestates, both by reducing odor and pathogen content. Ten full-scale biogas plants characterized by different plant designs (e.g. single digesters, parallel or serial digesters), plant powers (ranging from 180 to 999kWe), hydraulic retention time (HRT) (ranging between 20 to 70days) and feed mixes were monitored and odors and pathogens were observed in both ingestates and digestates. Results obtained indicated that MAD reduced odors (OU) from, on average, OU<inf>ingestate</inf>=99,106±149,173OUm^-2h^-1 (n=15) to OU<inf>digestate</inf>=1106±771OUm^-2h^-1 (n=15).Pathogens were also reduced during MAD both because of ammonia production during the process and competition for substrate between pathogens and indigenous microflora, i.e. Enterobacteriaceae from 6.85*10³±1.8*10¹ to 1.82*10¹±3.82*10¹; fecal Coliform from 1.82*10⁴±9.09 to 2.45*10¹±3.8*10¹; Escherichia coli from 8.72*10³±2.4*10¹ to 1.8*10¹±2.94*10¹; Clostridium perfringens from 6.4*10⁴±7.7 to 5.2*10³±8.1 (all data are expressed as CFUg^-1ww).Plants showed different abilities to reduce pathogen indicators, depending on the pH value and toxic ammonia content.